WASHINGTON – Veterans Affairs Secretary Eric Shinseki on Thursday defended an expensive proposal to extend disability payments to Vietnam veterans who get heart disease, saying studies show a significant link between the ailment and the toxic defoliant Agent Orange.

Responding to concerns about the costs of a rapidly expanding program, Shinseki told the Senate Veterans Affairs Committee that the law requires his agency to grant automatic benefits for diseases that are scientifically associated with Agent Orange. It leaves him no discretion to weigh costs or other considerations, he said, such as the fact that heart disease occurs commonly in men in their 60s and is strongly linked to smoking, poor diet and other factors.

Shinseki, a Vietnam veteran wounded in combat, said six of nine rigorous studies reviewed by the agency showed strong associations between the herbicide and heart disease. The evidence "more than satisfies" the law's requirements, he said.

Past studies on the link have been inconclusive, and the National Academy of Sciences says there is only a "modest" association, with no way to control for other risk factors. The VA acknowledges that the lack of data about soldiers' exposure to the defoliant makes it impossible to determine what role it may play in specific cases.

That has led some to question the proposal to add the disease — along with Parkinson's disease and certain types of leukemia — to a growing list of ailments qualifying for a program that Congress set up in 1991.

The agency estimates that the additions, set to take effect next month, could cost up to $67 billion in the next decade. The average veteran getting benefits for heart disease would receive about $1,000 per month, with many also getting new health care benefits.

Most lawmakers said Thursday they'll support the plan. But several raised concerns about covering common diseases and suggested the law be revisited.

"We have sometimes up here an uncomfortable duty to ask the hard questions," said Sen. Jim Webb, D-Va., a Vietnam combat veteran. He said there are too many unknowns about cause and effect, with little information specifically on exposure in Vietnam and no process for accounting for other risk factors.

Sen. Scott Brown, R-Mass., said Congress has a "moral obligation" to veterans "while also taking into consideration the financial obligations of these decisions."

The Associated Press reported last month that some 270,000 Vietnam veterans — more than one-quarter of the 1 million receiving disability checks — are getting compensation for type 2 diabetes, making it the most frequently compensated disability among the war's veterans.

Anthony Principi, the VA secretary under President George W. Bush who approved diabetes for the program, told lawmakers the agency needs more scientific guidance on the decisions. He also suggested a compromise in which the government could offer health care but not disability payments for illnesses where evidence is uncertain.